Neutral density (“ND”) filters are desirable in many applications. Generally, ND filters uniformly attenuate the intensity of light over a broad spectral range. Attenuation is accomplished by using any one or combination of several techniques. Light-absorbing glass is used in some ND filters, such as in sunglasses and photographic filters. Another type of ND filter uses a partially transparent thin-film metal coating that combines absorption and reflection. Both types of ND filters absorb light, which can cause heating in high light flux applications, as the absorbed light is converted to heat.
Another limitation of an ND filter using dyed glass is that the amount of attenuation is proportional to the amount of dye and the thickness of the glass. In many applications, the amount of dye is fixed, and the thickness of the glass is altered to provide the selected amount of attenuation. If a high degree of attenuation is required, the glass may become undesirably heavy and/or bulky. If a slight degree of attenuation is required, the glass may become undesirably fragile, or require additional support.
Another approach to providing an ND filter is to pattern a reflective metal layer on a dyed or undyed glass substrate. The metal-coated areas reflect a portion of the incident light, and the non-coated (clear) areas transmit another portion of the incident light. The amount of attenuation is controlled by how much of the surface of the filter is covered by the reflective metal layer. However, patterning the reflective metal layer, such as by photoresist lift-off or photoresist etching techniques, adds manufacturing complexity and yield loss. Patterning a reflective metal layer may have other undesirable effects, such as generating interference patterns in the transmitted light (particularly with a clear substrate) or creating an image artifact of the pattern. Finally, the remaining portions of the reflective metal layer can become quite hot in a high-flux environment.
If attenuation over a selected range of wavelengths is desired, a dielectric high-transmission filter is coated with a semi-transparent layer of metal or a patterned layer of metal. However, both techniques suffer the same problems that arise from using metal layers on dyed or plain glass substrates.
Therefore, an ND optical filter that avoids the aforementioned problems is desirable.